Propellers for aircraft and the like generally are designed in a manner such that only their outer portion contributes effectively to their intended use, which is usually for assisting in propelling aircraft and the like. In this connection, commonly only the outer portion of a propeller blade has an airfoil that is designed to contribute to the effective use of the propeller, whereas the inner portion of the propeller blade has a cross-section which contributes little to anything to the thrust resulting from the turning of the propeller. In the past the use of a propeller that had blades with an inboard portion of the blade that contributed little to the thrust of the propeller was unimportant since in many cases the engine adjacent to the propeller had a large frontal area and hence the effectiveness of the inner portion of the propeller blade was destroyed by this large frontal area anyway. However, with modern aircraft and modern engines, there is no such large frontal area which destroys the effectiveness of at least a portion of the inner portion of the propeller blade.
Not only does the common construction of a propeller blade prevent the inner portion of the propeller blade from being utilized for effective thrust, but also the design of the propeller blade can contribute significantly to drag which can absorb torque from the engine that is attached to the propeller. This drag occurs because the cross-section of the inner portion of the propeller blade does not have an effective air foil. Instead the common cross-section for the interior portion of a propeller blade would be generally oblong shaped or even circular shaped. Such a shape results in separation of the air flow from the propeller and results in undesirable drag. In addition, this drag increases with increased speed and hence is even more pronounced in modern high performance aircraft.
These undesirable effects of the common aircraft propeller blade and the like have been known for some years and some measures have been taken in the past to attempt to overcome these disadvantages. One method that has been utilized in the past has involved the use of what is termed a propeller cuff that can be located around and connected to the inner portion of a propeller blade. This propeller cuff is designed to give the exterior of the inner portion of the propeller blade an appropriate airfoil cross-section so that the blade with the propeller cuff will give thrust not only from its outer portion but also from the inner portion which has the connected propeller cuff. Such a propeller cuff is set forth in U.S. Pat No. 2,289,400.
Although propeller cuffs have been used in the past, their use has been limited. Commonly, modern propellers have blades whose pitch is variable so that the pitch can be varied depending upon the desired flight conditions. Unfortunately, a propeller blade which has a cuff will generally result in drag occurring as a result of the junction between the outer surface of the propeller spinner and the adjacent inner surface of the propeller cuff. Usually there is a significant gap at this juncture and this results in air turbulence and associated drag. Various attempts have been made in the past to reduce this gap to minimize the turbulence and associated drag; however, with modern variable pitch propellers, it has not been possible to achieve a minimum gap over a wide range of blade positions. In one instance the inner end portion of the cuff was designed so that it presented a minimum clearance when the propeller blade and associated cuff was in its normal cruise position; however, this did not result in the elimination of drag when the propeller was rotated to its other positions.
Various attempts have been made to solve the problems associated with the use of propeller cuffs in connection with variable pitch propellers. Some of these attempts are set forth in U.S. Pat. Nos. 2,289,400 and 2,408,677. However, in these arrangements the range of the pitch of the blade is limited without incurring appreciable drag between the inner edge of the cuff and the outer surface of the propeller spinner, and in one instance an indentation in the propeller spinner is employed which would be impractical to use with modern day spinners. Insofar as the arrangement in U.S. Pat. No. 2,289,400 is concerned, it is clear that the range of pitch of the propeller must be limited in view of the construction of the propeller fairing. Insofar as the embodiments of the structure set forth in U.S. Pat. No. 2,408,677 are concerned, it is clear that the embodiments set forth in FIGS. 1 and 2 would require the range of the pitch of the blade to be restricted in order that a significant gap would not exist between the lower portion of the cuff and the associated projection on the propeller spinner. In addition, the outer flat shape of the projection departs significantly from the adjacent shape of the spinner and hence can create undesirable turbulence. The other embodiment illustrated in FIG. 3 of this patent is more desirable in that it eliminates this outer flat surface and instead substitutes a depression into the spinner itself. However, such a construction of the propeller spinner is impractical and may also limit the range of the variable pitch of the associated propeller blade.
In the past, the deficiencies associated with prior attempts to reduce drag on variable pitch propellers with cuffs and the like have not been deemed to be that significant since the associated drag could be overcome by supplying additional engine power. However, at the present time with the high cost associated with aircraft fuels and the like, even minor reductions in drag can result in significant cost savings over an extended period of time. Not only would a reduction in drag between the spinner and the associated propeller cuff reduce the amount of fuel that had to be consumed for a given flight and hence result in a cost savings, but it also might enable the aircraft to carry less fuel and hence less weight for a given flight or increase its range. Consequently, it is now extremely important that any unnecessary drag between a propeller spinner and the associated propeller cuff be eliminated insofar as it is practical and does not interfere with the proper operation of the variable pitch propeller.
The present invention overcomes these problems associated with the drag that occurs between a propeller spinner and the adjacent edges of a propeller cuff on a variable pitch propeller and provides propeller apparatus in which a variable pitch propeller and associated propeller cuffs are movable through a wide range of propeller pitches with the inner portion of the propeller cuff maintaining a uniform narrow gap with the adjacent exterior portion of the propeller spinner. Moreover, this invention does not require that there be any unusual indentations or the like in the propeller spinner itself.